National & American League Professional Baseball Clubs v. Major League Baseball Players Association

Players Andy Messersmith and Dave McNally (plaintiffs), having refused to sign new contracts after the 1974 season, played the 1975 season under the Uniform Player's Contract's one-year renewal option, and MLB clubs (defendant) argued that this same reserve clause carried forward into the renewed contract, letting clubs renew a player's contract indefinitely and keep him locked in MLB's reserve system barring negotiation with other teams. The players argued the reserve clause could be exercised only once, making them free agents after the one-year renewal, and their union filed a grievance with an arbitrator.